Highly versatile aliphatic polyesters have been the focus of attention as biodegradable plastics, and polylactic acid (PLA) polybutylene succinate (PBS), polyethylene succinate (PES), and polycaprolactone (PCL) and the like are commercially available.
One application for these biodegradable aliphatic polyesters is the field of films for packaging, agriculture, and foods and the like, and high strength, thermal resistance, gas barrier properties, and biodegradability are basic required characteristics, depending on the application.
Of the aforementioned aliphatic polyesters, PLA consists of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA). The melting point of these individual crystals (α crystals) is approximately 170° C. and the thermal resistance may be insufficient when compared to polyethylene terephthalate and the like, so there is demand for improvement.
On the other hand, various methods of forming a stereocomplex by kneading poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA) have been proposed as a method for further improving the thermal resistance of PLA (for examples, patent document 3, patent document 4, and non-patent document 1).
These stereocomplexes (SC) are eutectoid crystals of poly-L-lactic acid (PLLA) and poly-D-lactic acid (PDLA), and the melting point of these crystals is approximately 50° C. higher than the α crystals, and utilization of this property is anticipated.
However, even if a film is formed from a composition obtained simply by melting and kneading PLLA and PDLA, a stereocomplex will not easily be formed, and although the film that is formed will have improved thermal resistance, the film will be brittle and use as a packaging film will be difficult.
Therefore, the present inventors have proposed that an stretch film with excellent thermal resistance and toughness can be obtained by at least uniaxially stretching at specific conditions a composition obtained by melting and kneading PLLA and PDLA (Patent application 2004-146239).
This stretch film has a wide angle x-ray diffraction peak (2θ) around 16° (Hereinafter a peak detected in this region will be referred to as (PPL).), and the total area (SSC) of the diffraction peaks (2θ) in the vicinity of 12°, in the vicinity of 21°, and in the vicinity of 24° (Hereinafter the peaks detected in these regions will be jointly referred to as (PSC).) is less than 10% with respect to the sum of (SSC) and the area (SPL) of the diffraction peak (PPL) in the vicinity of 16°.
Therefore, the SC crystals in the stretch film are more rare than the individual crystals of PLLA and PDLA.
Furthermore, the present inventors have proposed a method for manufacturing a stretch film primarily consisting of SC crystals with major wide angle x-ray diffraction peaks (2θ) in the vicinity of 12°, in the vicinity of 21°, and in the vicinity of 24°, by performing a specific heat treatment on the stretch film (Patent application 2004-146240).
Furthermore, methods for forming a layer of inorganic oxide, inorganic nitride, or inorganic nitride oxide have been proposed as a method for improving the gas barrier properties of polylactic acid based biaxial stretch films (patent document 5). However, these vapor deposition methods or the like are difficult and expensive to implement, and the vapor deposition film is extremely thin, so control of the barrier properties or the like has been a problem.    [Patent document 1] Japanese Patent Application Laid-open No. H7-207041    [Patent document 2] Japanese Patent Application Laid-open No. H8-198955    [Patent document 3] Japanese Patent Application Laid-open No. H8-25400    [Patent document 4] Japanese Patent Application Laid-open No. 2000-17164    [Patent document 5] Japanese Patent Application Laid-open No. H10-24518    [Non-patent document 1] Macromolecules, 20, 904 (1987)